2- aza-l,4-dienes

Covalent fluondes of group 3 and group 5 elements (boron, tin, phosphorus, antimony, etc ) are widely used m organic synthesis as strong Lewis acids Boron trifluoride etherate is one of the most common reagents used to catalyze many organic reactions. A representative example is its recent application as a catalyst in the cycloadditions of 2-aza-l,3-dienes with different dienophiles [14] Boron trifluoride etherate and other fluonnated Lewis acids are effective activators of the... 

Carbonylation by CDI of 2-aza-l,3-dienes yields 4-(l.H)-pyridones11201 (see also Chapter 14) ... 

Ketenes, which are even more reactive than isocyanates, afford ketenimines at or below room temperature [62CRV247 84JOC2688 89JCS(P1)2140]. At elevated temperatures, dimerization or polymerization occurs (21HCA887). Although N-aryl- and A-vinyliminophosphoranes react smoothly with ketenes, strong acceptor substituents on the nitrogen hamper the reaction thus V-acyliminophosphoranes do not react with ketenes. Vinylketenimines such as 2-aza-l,3-dienes prepared in this way from... 

The Lewis acid-mediated reactions of 2-aza-l,3-dienes and aldehydes, resulting in tetrahydro-l,3-oxazin-4-one derivatives, were explained in terms of the competitive existence of two reaction pathways a [4+2] hetero-Diels-Alder cycloaddition reaction and a Mukaiyama aldol reaction <2001TA439>. 

Fleury and co-workers (87HCA187) reported that somewhat related 2-azadienes 136, 4-(dialkylamino)-l,l-dicarbonitrile-2-aza-l,3-diene and their 1-methoxycarbonyl analogues, afforded imidazoles 137 when reacted with N-substituted or N,N-disubstituted hydrazines and hydrazides by double nucleophilic attack. Moreover, the addition of cyanide to 136... 

The 5,7-diaryl-2-fluoro-4/7-l,3-diazepines have been synthesized from 3-aryl-substituted 277-azirines and difluoro-carbene (Scheme 13). The reaction involves isomerization of azirinium ylide into a 2-aza-l,3-diene, which undergoes [4+2] cycloaddition with the starting azirine, followed by ring expansion and dehydrofluorination <2006TL639>. 

Armesto, D., Hoorspool, W.M., Ortiz, M.f., and Romano, S. (1992) Reaction of anions from monoimines of benzil with alkylating agents. Photochemical reactivity of some 4-alkoxy-2-aza-l, 3-dienes. Journal of the Chemical Society, Perkin Transactions 1, 171-175. 

Aza-1,3-dienes. The reaction of imines (1) of aldehydes obtained from bis(trimethylsilyl)methylamine with aldehydes catalyzed by Bu4NF (Peterson alkenylation) provides 2-aza-l,3-dienes in 65-78% yield. 

Aza-l,3-dienes, l,4-bis(dimethylamino)-, pyrido- and pyrimido-fusion using,... 

Aza-l, 3-dienes, 1,1-bistrifluoromethyl-, cycloaddition to diazomethane, 60, 39 2-Aza-1.3-dienes, 3-carboxy-, formation, dimerization, 57, 43 2-Aza-l, 3-dienes, 3,4-dicarboxy-, formation, cycloadditions, 57, 43 2-Aza-l, 3-dienes, 4-dimethylamino-3-methoxycarbonyl-1, l-di(methylthio)-, 57, 29... 

Aza-l,3-dienes react with diazenedicarboxylates and 4-phenyl-3ff-l,2,4-triazole-3,5(4//)-dione to afford in good yield, l,2,3,6-tetrahydro-l,2,4-triazines 14 and 15, respectively. The relative configuration of the substituents at C-3 and C-6 of one product derived from diethyl diazenedicarboxylate was ascertained by X-ray analysis32. 

To 5 mL of a benzene solution of 5 mmol of 2-aza-l, 3-diene are added 5 mmol of the dialkyl diazenedicarb-oxylate. The reaction mixture is stirred at 25 C for 10 h. Then the solvent is removed (0.1 Torr) and the resulting residue is washed with CH3OH, giving a white solid which is recrystallized from hcxanc/CHCl,. 

To 25 mL of a CH,C12 solution of 3 nimol of 2-aza-l,3-diene are added 3 mmol of 4-phenyl-3//-1,2,4-tria-zole-3,5(47/)-dione. The red color disappears after a few minutes and the product is isolated and purified as described in the previous procedure. 

This synthetic approach, however, fails with a 2-aza-l,3-diene, ( ,is)-4-benzyl-3-tm-butyldi-mcthylsilyloxy-l-isopropoxy-2-aza-l,3-butadicnc, since in this case the 1,2,5-oxadiazine adduct is obtained in only 25% yield and cannot be separated from the reaction mixture104. 

In contrast, treatment of the related 277-azirine-3-methylacrylate 785 with imidazoles and pyrazoles gave 2-aza-l,3-dienes 786 derived from a transient addition product (Scheme 189) <1999JOC49>. These dienes are useful in hetero-Diels-Alder reactions with electron-deficient dienophiles. 

Heteroarylamines, for example 1195, react with (dimethylamino)propenoate 1196 to yield an imidazolecarboxylate 1199. The imidazole ring is formed via the intermediate diaminoalkenoate 1197, which undergoes an intramolecular Michael addition followed by a retro-aldol-like reaction (Scheme 294) <1998JHC1527>. Similarly, 4-dimethyl-amino-2-aza-l,3-dienes 1200, serving as y-dielectrophiles, condense with amines or hydrazines neat at 70 °C to form A -substituted imidazole-4-carboxylates 1201 in 60-75% yields (Scheme 294) <1999TL8097>. 

Aza-l,3-diene can be synthesized by treatment of an aldehyde with N,N-bis (trimethylsilyl)enamine in the presence of CsF. The reaction eliminates siloxane without forming water. This is an extremely useful means of producing such acid-sensitive compounds (Scheme 2.16) [31]. 

C cloadditlon of the electrophilic dienophile 1,4-naphthoquinone with a 2-aza-l,3-diene provides access to the benzo[g]isoquinoline system (F. Sainte et al, J. Amer. chem. Soc., 1982, 1, 1428). 

Intermolecular cyclization of 4-dialkylamino-2-aza-l,3-diene-1,1-dicarbonitriles proceeds on treatment with hydrogen bromide in acetic acid to yield 2-bromo-3-cyanopyrazine <90HCA1210>. Tetraalkyl or -arylpyrazines are synthesized by reductive cyclization of the corresponding nitriles with titanium tetrachloride and zinc in modest to good yields (Equation (18)) <90S70l>. 

The incorporation of a 2-aza-1,3-diene into reactive systems including N-arylketenimines,653,66 N-arylvinylketenimines,656,66 aryl and vinyl isocyanates,67,68 vinyl isothiocyanates,68 and vinyldiimides69 has been shown to provide 2-aza-l,3-diene systems capable of participating in [4 + 2] cy-... 

N-trimethylsilyldivinylamines can be prepared stereoselectively by reacting 2-aza-l,3-dienes with trimethylsilyl... 

Triphenyl azapentadiene 19 treated with PTAD gives Diels-Alder adduct 20 (75CJC355). Azapentadiene 21 reacts with PTAD via the Diels-Alder adduct 22, which, in this case, cannot be isolated, giving bicyclic lactam 23 (87HCA1255), The usual Diels-Alder reaction has been described for some other 2-aza-l,3-dienes as well [89JCR(S)66]. Diels-Alder reaction of PTAD with diazadiene 24 provides 25 [82IJC(B)589],... 

Under solvent-free conditions with microwave irradiation. Moody et al. have improved the hetero Diels-Alder reaction of eneacylamines with l-alkoxy-2-aza-1,3-dienes [67]. A mixture of l-(2-thiazolyl)-l-acetylaminoethylene 54 and the starting 2-aza-l,3-diene 55, irradiated in a GEM Focused Synthesizer at 180 °C for 15 min produced a 64% yield of adduct 56 (Scheme 11.16). Carboxylic acid esters and thiazole rings are tolerated under these reaction conditions (Scheme 11.16). Without microwave irradiation yields were in the range 25 to 42% [67]. 

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